This invention is directed for use in underwater searching or exploring. The device is used principally for locating and viewing the bottom of a body of water and for searching for objects, usually fish, in the body of water. However, for simplicity the device will usually hereinafter be referred to as a depth finder but with the understanding that it can serve other functions as well.
In one known type of depth finder the display is in the form of a bar-graph on a strip chart produced by a stylus inking or burning in markings, which represent echoes or returns, on a paper strip. Briefly, in this type of paper-stylus or strip chart device, a burst or pulse of sonar energy is emitted in a beam from a transducer fixedly located on the vessel. The stylus makes a vertical sweep across the paper strip for each transmitted pulse and when an echo is received the stylus makes some visible mark on the strip of paper. The strip moves transversely which is somewhat representative of the movement of the vessel. The stylus makes its vertical travel for each transmitted pulse and makes a visible marking for each received echo. This then forms what is known as a bar-graph type pattern for the viewer. Assuming generally that the transducer is directed to beam its pulse downward, the produced visible pattern then gives the viewer a general overall view of the area below the vessel as the vessel passes over it. The vertical sweep of the stylus represents the depth or range from the vessel and the stylus oscillates vertically at about the pulse repetition rate of the transmitted pulse.
Another known type of depth finder produces a display on a CRT but in a PPI type of pattern. In this device the transducer sweeps back and forth to transmit the sonar bursts over an area and the received echoes are then displayed as brightened areas on the face of the CRT. This type of depth finder and display gives the viewer an overall view of the area below, in front, behind, or alongside, the vessel. In this type of device, as the transducer continues to sweep the area being explored, returns or echoes are continuously being received to update the presentation of detected objects in the area being searched. In other words, where a target is viewed on the screen it means that the object is continuing to produce an echo as the beam of sonar energy sweeps across it. When the object is no longer being struck by the transmitted sonar pulse or the reflection is too small or too distant to make a showing on the CRT, it no longer appears on the CRT. This is distinguished from the paper-stylus bar-graph type display. In the latter when an echo is received it is permanently recorded even though the object might have since moved or disappeared.
Conventionally, depth finders have a transceiver which transmits a sonar or ultrasonic signal through a transducer into the body of water and the echoes or returns reflected back from the bottom of the body of water and objects in the water are picked up by the transducer and amplified in a receiver. To date the sonar depth finders which provide a video or visual presentation of the signal returns in bar-graph form have used a moving paper (or similar) graph and a stylus which inks or burns-in a pattern on the graph paper corresponding to the occurrence of returns or echoes from the transmitted pulses, as described above. This type of mechanical reproduction for video presentation of the returns has a number of drawbacks. For one, because of the relatively slow speed of operation of the stylus, the pulse repetition rate of the transmitted signal is limited and is virtually fixed whereas at times it might be useful to have a faster pulse repetition rate in order to get a more distinct view of the area being searched. Furthermore, the mechanical movement of the stylus and the continuing movement of the graph paper can result in malfunctions especially since the units are often used in an environment where the humidity is high and the device may be located in a salt atmosphere. In addition, because it is necessary from time to time to replace the rolls of chart paper and to adjust the stylus, the mechanical units must be accessible and this can lead to mishandling and damage to the unit.